feat: add code coverage with branch coverage

- Add --enable-coverage configure option
- Add lcov targets for generating coverage reports
- Enable branch coverage (--rc branch_coverage=1)
- Install lcov in all Docker images
- Coverage report displayed via lcov --summary (no Codecov - doesn't work with Gitea)

Dockerfile.debian

@@ -12,6 +12,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   ccache \
   gcc \
   git \
+  lcov \
   libcmocka-dev \
   libcurl3-dev \
   libgcrypt-dev \